Numerous types of process instruments are available for measuring level, such as level in a tank. One such instrument is a displacement servo gauge. A servo gauge uses a counterweight or spring to balance the weight of a displacer. The displacer is suspended on a cable from a drum. The drum is coupled to the spring or counterweight. The spring or counterweight is selected so that an equilibrium condition exists when the displacer is at the top surface of the liquid. A change in the liquid level causes a corresponding change in the counterforce to move the gauge out of balance. Upon sensing such an imbalanced condition, a servo motor rotates the drum to raise or lower the displacer, as is necessary, until the gauge is again in balance. Rotational movement of the motor, which represents movement of the drum and thus the cable and displacer, is sensed to calculate the change in level.
Problems have been found associated with prior displacement servo gauges. Particularly, errors in level measurement can occur due to variations in drum diameter, as well as weight of the cable. Specifically, if the cable is wound in multilayers on the drum, then the exact length of cable relating to an increment of rotational movement of the servo motor shaft continually changes. This is due to the difference in circumferential length of the outer layers versus the inner layers when wound on the drum. Further, in the equilibrium state, the applied forces are balanced when the sum of the weight of the cable and the weight of the displacer is equal to the buoyancy force exerted by the liquid and the counterforce of the gauge. The weight of the displacer is a constant. In conventional gauges the counterforce weight is also constant. However, the weight of the tape varies with its extended length, which in turn varies with the liquid level. Thus, with conventional gauges, any change in weight of the tape requires a variation in the buoyancy by changing the penetration of the displacer and the liquid. This change results in an error in the reading of the level of the fluid, reflecting this change of penetration.
In certain applications, knowing the specific gravity of the liquid is important. In fact, the specific gravity has an effect on the operation of the gauge. Therefore, it is desirable to quickly and easily measure the specific gravity of a liquid, the level of which is being sensed.
Conventional servo gauges measure a variation of torque in the drive gear train between the servo motor and the drum. An alternative approach is described in Schmidt et al. U.S. Pat. No. 4,236,314 which discloses the motor pivotally suspended from a shaft of the cable spool. Therefore, the motor itself acts as a counterweight. A switch is positioned within the range of pivotal movement of the motor and is actuated when a displacer impinges on the surface of the liquid. With such a construction, the position of the switch is important relative to the particular liquid being sensed. Specifically, changes in specific gravity of the liquid require changes in positioning of the switch due to a corresponding change in the buoyancy force. Moreover, such a construction, as well as the conventional construction, permits sensing of only a single type of liquid.
Frequent calibration of level gauges has proven to be both costly and difficult. Normally, a maintenance person must go out to the site of the instrument to manually measure the liquid and calibrate the gauge. Where the gauge is mounted at the top of the tank, this proves quite cumbersome.
The present invention is directed to solving one or more of the problems discussed above.